1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up an image of an object.
2. Related Background Art
In the image pickup apparatus constituted by a CCD (charge coupled device) there has conventionally been adopted a control method, as shown in FIGS. 1 to 3, of supplying the CCD with a field reading substrate voltage (VSUB) in case of field readout in which two pixels adjacent to each other in the vertical direction are added to be read, and with a frame reading substrate voltage in case of frame readout in which the pixels in the vertical direction are read one by one without adding of two pixels.
In another control method, the substrate voltage supplied to the CCD is maintained same both in the frame readout and the field readout.
An example of changing the substrate voltage for the two scanning method is disclosed in the Japanese Patent Publication No. 01-238385.
FIG. 4 is a cross-sectional view of a unit pixel of a CCD disclosed in the Japanese Patent Publication No. 7-93706, wherein shown are a photodiode (PD) 71, a transfer gate area 72, a ½-step vertical transfer CCD (V-CCD) 81, a P-well 82, a channel stop 83, an n-type substrate (n-SUB) 84, an inverse bias voltage (VSUB) 85 applied between the P-well and the n-substrate, and incident light 86. In this unit pixel, there is adopted a transfer gateless electrode configuration in which the V-CCD and the transfer gate area are driven by a common transfer electrode.
The Japanese Patent Publication No. 7-93706 discloses switching the inverse bias voltage (substrate voltage) VSUB between a first scanning method of reading the entire image area in a field period by simultaneously reading the signals of two adjacent horizontal pixel rows by one horizontal scanning and a second scanning method of scanning the entire image area in two fields (one frame) period by reading a signal of a horizontal pixel row by one horizontal scanning of the interlaced scanning method.
However, in the CCD constituting the image pickup apparatus, a change in the substrate voltage varies the peak value of the barrier potential for charge to be discarded toward the substrate. A higher substrate voltage shifts the peak value of the potential shallower, thereby reducing the charge amount that can be accumulated in each pixel. A lower substrate voltage shifts the peak value of the potential deeper, thereby increasing the charge amount that can be accumulated in each pixel. Also a shift of the peak position of the potential to the shallower side reduces the sensitivity to red colored light of longer wavelength, and a shift of the peak position to the deeper side increases the sensitivity to red colored light of longer wavelength.
In consideration of the above-described property of CCD, in order to obtain a substrate voltage suitable for the field signal readout in which the two pixels are added in the vertical transfer channel (signal transfer channel) of CCD, it is necessary to elevate the substrate voltage thereby obtaining a shallower peak value of the barrier potential to reduce the saturation accumulated charge amount per pixel, whereby the charge amounts of two pixels at the addition thereof in the vertical transfer channel do not exceed the saturation charge amount of the vertical transfer channel. If such substrate voltage suitable for field signal readout is applied to the frame signal readout, the saturation level of the CCD output becomes lower because the saturation accumulated charge amount per pixel is made smaller, whereby the dynamic range becomes narrower.
On the other hand, if the substrate voltage is lowered for the frame signal readout, thereby shifting the peak of the barrier potential to the deeper side and increasing the accumulated charge amount per pixel and if such substrate voltage is used in the field signal readout, the charge amount of the two pixels at the addition thereof in the vertical transfer channel may exceed the saturation charge amount thereof, thus resulting a blooming phenomenon caused by the overflowing of the charge into the signal of other pixels.
The limited dynamic range and the blooming phenomenon mentioned above can be avoided by the use of the substrate voltages respectively matching the field signal readout and the frame signal readout, while the spectral sensitivity becomes different between the field signal readout and the frame signal readout because of the difference in the substrate voltage, whereby the color reproducibility and the luminance level of the finally obtained image change.
Also in the conventional CCD, in addition to the control method of supplying the CCD with a field reading substrate voltage in case of field readout in which two pixels adjacent to each other in the vertical direction are added and read, and with a frame reading substrate voltage in case of frame readout in which pixels in the vertical direction are read one by one without adding of adjacent two pixels, the sensitivity of the field reading method at the CCD reading is assumed to be twice of that of the frame reading method, and the exposure for obtaining a still image in the frame image readout is controlled by the field readout method, based on the exposure amount detected by the field reading method and on the assumption that the sensitivity of the frame readout method is ½ of that of the field readout method.
FIG. 5A is a cross-sectional view of a CCD, and FIG. 5B is a view showing the electronic potential in the horizontal and vertical directions along a broken line 5B—5B in FIG. 5A. In FIG. 5A, there are shown an n-type semiconductor substrate 21, a p-type first semiconductor area 22, an n-type photoelectric converting element area 23, an n-type channel area 24 of a vertical charge transfer element having an o2 area, a p-type channel stop area 25, a transfer electrode 26 of a vertical charge transfer element adapted to transfer the photoelectrically converted charge of the n-type channel area of the o2 area at a potential d and to intercept such charge at a potential e, and a power supply 27 for applying a substrate voltage (VSUB) to the n-type semiconductor substrate 21.
Referring to FIG. 5A, if the substrate voltage (VSUB), applied to the n-semiconductor substrate 21 through the power supply 27 is lowered from a voltage corresponding to a barrier a in the CCD as shown in FIG. 53, the barrier tends to increase as indicated by b in FIG. 5B. This barrier inhibits the migration of the charge accumulated by photoelectric conversion in an area o1, toward an area o3. On the other hand, increase in the substrate voltage facilitates the electron flow as indicated by c. Thus, increase in the substrate voltage shifts the peak of the barrier to a shallower position as indicated by a to c, whereby the charge amount that can be accumulated in each pixel decreases. On the other hand, decrease in the substrate voltage shifts the peak of the barrier to a deeper position as indicated by a to b, whereby the charge amount that can be accumulated in each pixel increases.
Such shift of the potential peak position toward the shallower position, caused by the lower voltage applied to the substrate, decreases the sensitivity to the light of longer wavelength or red color region, while the shaft toward the deeper position, caused by the higher voltage applied to the substrate, increases the sensitivity to the light of longer wavelength or red color region.
If a pulse-shaped voltage (ESH) is applied to the n-semiconductor substrate 21, the barrier shifts from a to c, whereby all the charge in the area o1 can be transferred to the area o3. Such function can be utilized to arbitrarily set the start time of charge accumulation in the area o1, and such pulse-shaped voltage is called an electronic shutter.
In consideration of the above-described property of CCD, in order to optimize the substrate voltage to the field readout in which two pixels are added in the vertical transfer channel of the CCD, it is necessary to elevate the substrate voltage, thereby realizing a shallower potential peak and decreasing the saturation accumulated charge amount per pixel, whereby the charge amount of the two pixels does not exceed the accumulation charge amount of the vertical transfer channel at the addition of two pixels therein. If the substrate voltage optimized to the field readout is used in the frame readout, there is raised a problem that a saturation CCD output is smaller, since such the substrate voltage is set to provide smaller saturation accumulated charge amount per pixel.
On the other hand, if the substrate voltage is lowered for optimization for the frame readout, thereby shifting the potential peak deeper and increasing accumulated charge amount per pixel, and if such substrate voltage is used for the field readout, the charge amount of two pixels at the addition thereof in the vertical transfer channel exceeds the accumulation charge amount thereof, and the overflowing charge is mixed with the charge of other pixel signals, thus causing so-called blooming phenomenon.
Based on these facts, the substrate voltage is switched to the optimum values respectively for the field readout and the frame readout.
In picking up a still image of the frame readout method with the above-described control of the substrate voltage, the exposure conditions such as the exposure time and the diaphragm aperture for the still image are determined by a signal read out in the field readout method. In the determination of such exposure conditions, it is assumed that the signal amplitude in the field readout method, in which two pixels are added in the CCD, is twice of that in the frame readout method in which the signal is read pixel by pixel. However, in such drive control, the substrate voltage in the frame readout method is different from and is lower than that in the field readout method whereby the potential peak is the CCD is deeper and the sensitivity becomes higher for the long-wavelength red color region. The image obtained by the frame readout therefore shows a difference in the luminance, corresponding to the difference in the sensitivity in the long-wavelength region, from the exposure conditions determined in the field readout method, whereby the image of the optimum luminance cannot be obtained.
Also the color reproducibility of the image becomes different between the field readout method and the frame readout method, because of the change in the sensitivity of the CCD in the longer wavelength region.